Conventionally, as a color image forming apparatus which uses an electrophotography method, an apparatus which uses a plurality of developers for one photosensitive body to develop respective color components is known. This apparatus repeats an “exposure—development—transfer” process a number of times equal to the number of color components to overlay and form color images on a single transfer sheet in these processes, and to fix these color images, thus obtaining a full-color image.
With this method, the image forming process must be repeated three times, or four times if black is used, per print image, and thus takes a long time to complete image formation.
As a method that can cover this shortcoming, a technique that uses a plurality of photosensitive bodies, overlays visible images obtained for respective colors in turn on a transfer sheet, and obtains a full-color print by a single sheet feed process is known.
With this method, the throughput can be greatly improved. However, color shifting owing to position shifts of respective colors on the transfer sheet occurs due to limitations on the achievable positional precision of, and diameter shifts (slight shifts in position of the axes) of the respective photosensitive bodies, positional precision shifts of the respective optical systems, and the like, and it becomes difficult to obtain a high-quality full-color image.
As a method of preventing this color shifting, a technique for forming a test toner image on a transfer sheet or a conveyor belt that forms a part of transfer means, detecting that image, and correcting the optical paths of respective optical systems and correcting the image write start positions of respective colors based on the detection result is known (e.g., Japanese Patent Laid-Open No. 64-40956; to be referred to as “reference 1” hereinafter).
Also, a technique for automatically converting the output coordinate positions of image data for respective colors into those from which any registration shifting is corrected, and correcting the positions of modulated light beams in an amount smaller than a minimum dot unit of each color signal by correction means on the basis of the converted image data of the respective colors is known (e.g., Japanese Patent Laid-Open No. 8-85237; to be referred to as “reference 2” hereinafter).
However, with the technique of reference 1, the following problems remain unsolved.
First, in order to correct the optical paths of the optical systems, a correction optical system including a light source and f-θ lens, mirrors in the optical paths, and the like must be mechanically operated to adjust the position of the test toner image. That is, high-precision movable members are required, resulting in high cost. Furthermore, since it takes a long time to complete the correction, the correction cannot be frequently made. Also, the optical path lengths often change with the lapse of time due to the temperature rise of the machine. In such case, it is difficult to prevent color shifting by correcting the optical paths of the optical systems.
Second, upon correcting the write-start positions of images, the position shifts of the upper end and upper left portion can be corrected. However, any tilt of an optical system, and any magnification shifting that may occur due to possible optical path length shifting, cannot be corrected.
In reference 2, as a result of correcting the output coordinate positions of image data for respective colors for an image that has undergone halftone processing, dot reproducibility of the halftone image deteriorates, color nonuniformity occurs and moire becomes obvious.
FIG. 1 shows an example, which will be described below. An input image 101 has an image having a given density value. Assume that an image 102 obtained by applying arbitrary color shifting correction to this input image 101 is printed in practice. In this case, since the image density values and toner densities for that image density value have a nonlinear relationship, although the input image 101 has a constant density value, if the image after color shifting correction is printed, the result is an actual printed image whose density value is not constant. Therefore, when such nonuniform density values appear periodically, moire becomes obvious, and a high-quality color image cannot be obtained.
Furthermore, in the search for ways to speed up printer engines, it has become common not to stop the photosensitive drum during scanning exposure of a laser beam, but rather to rotate it even during scanning exposure. At this time, if the scanning exposure directions of image forming units of respective color components are the same, no problem is posed. However, when a given image forming unit scans in a direction opposite to that of another image forming unit, this causes color nonuniformity. Since the scan speed and rotational speed of the drum vary depending on print mode, color shifting cannot be suppressed by means of a single countermeasure processing so far.